The present invention relates to a photosensitive recording material comprising a support and a negative-working photosensitive layer which has a rough surface and which contains a diazonium salt polycondensation product as the photosensitive compound.
Presensitized printing plates, the surfaces of which have a particular degree of roughness, are known in the art. These plates offer the advantage of considerably reducing the vacuum contact time, i.e., the time which is required to evacuate the vacuum frame, until an entirely uniform contact between the original and the printing plate is obtained. With these plates it is also possible to prevent the formation of air pockets between the original and the printing plate.
In most cases, a rough surface is produced by coating the photosensitive layer with an additional layer which is roughened or rendered matt, respectively, by adding pigment particles, by embossing with the aid of a correspondingly roughened surface, for example, with an embossing roll, or by any other suitable means. In the subsequent developing process, the matted covering layer is dissolved in the developer. Materials of this kind are described in German Offenlegungsschriften No. 25 12 043 and No. 26 38 710. It is a disadvantage of these plates that they must be coated in two steps and that the entire substance of the covering layer, including any pigment particles which may be present, collects in the developer, which is thus rendered useless more rapidly.
In German Offenlegungsschrift No. 29 26 236, similar recording materials with rough surfaces are described, in which the photosensitive layer contains particles having a diameter which is greater than the thickness of the layer. If the difference between particle diameter and layer thickness is in a particular range, a corresponding distance between the surface of the layer and the original used for copying is obtained. Since the exposure lamps which are customarily used never emit ideally parallel light, slightly unsharp reproduction in the layer is to be expected in this case, i.e., scattered light passes into boundry areas of the photosensitive layer that would otherwise be covered by an original which would be in close contact with the layer. Depending on the size, index of refraction, and quantity of the particles contained in the layer, similar scattering, refraction, or even diffraction phenomena may also be caused by the particles themselves.
All these effects can result in a reduction of the (unexposed) screen dots which remain on the support in the development of positive-working photosensitive layers. Although these exposure effects are not clearly understood, it has been found that, in the exposure and development of a photosensitive material according to German Offenlegungsschrift No. 29 26 236, demonstrable differences actually occur in the surface coverage of the developed printing plate, depending on whether the photosensitive layer does or does not contain pigment particles.
In the table below, the percentage surface coverage, i.e., the percentage of a surface covered by image elements, is compared for (1) an original film, (2) a printing plate containing particles in the photosensitive layer, which has been exposed under the original film and then developed, and (3) a printing plate without particles in the layer, which has been processed in the same way.
TABLE 1 ______________________________________ Surface Coverage in % Film 95 89 81 72 63 54 47 38 29 20 12 5 ______________________________________ Positive 99 97 92 86 80 71 62 53 43 31 21 10 layer with particles Positive layer 99 98 94 89 83 74 66 57 47 34 23 11 without particles ______________________________________
As the data in Table 1 show, the layer containing pigment particles has a surface coverage, i.e., a visually appearing optical density, which is noticeably closer to the density of the original than that of the corresponding layer without particles; in practice, the tonal rendering of the layer which contains particles is referred to as being "lighter" (showing reduced screen dots). This result is desired and can be regarded as an additional advantage of the positive-working materials according to German Offenlegungsschrift No. 29 26 236.
Based on this knowledge, it was to be expected that an opposite effect would necessarily occur in the case of negative (reverse-working) layers, i.e., tonal rendering of the copy was expected to increase. Since negative layers also yield prints in which tonal rendering is invariably denser than in the original from which the negative master has been produced, one actually had to be prepared for a deterioration in the quality of the print obtained from a negative-working printing plate of the above-described kind.
In fact, a comparison between (1) a known negative-working printing plate in which a covering layer containing particles that produce a roughening effect is applied on top of the photosensitive layer and (2) an otherwise identical plate which lacks pigment particles shows that the printing plate obtained from the pigment-containing plate prints "heavier" (with larger screen dots). In other words, plate (1) yields a print having a higher surface coverage than a print obtained from the printing plate (2) which omits the particles.
Moreover, it was expected that, in a negative plate having a photosensitive layer containing pigment particles of a diameter exceeding the layer thickness, the layer areas underneath a particle would not be hardened or would at least be incompletely hardened upon exposure, because the particles absorb, reflect, or scatter the light. Since in a particle-containing layer the particles remain in the printing image areas of the layer, it was also expected that the particles would not be anchored firmly enough and would consequently break out after printing a relatively small number of copies, thus rendering the printing plate useless.
It is probably for these reasons that, to date, only those printing plates roughened by particles have been accepted in practice that either work positively or contain the particles in a covering layer which is applied on top of the photosensitive layer and which is removed together with the particles in the developing process. The few publications which describe negative-working layers containing pigment particles generally relate to printing plates which do not require a high resolution, for example, photopolymerizable relief printing plates, as described in German Offenlegungsschrift No. 24 03 487.